SEOUL, Korea, Republic Of -- As world leaders debated how to punish North Korea's claim of a fourth nuclear test, South Korea retaliated by broadcasting anti-Pyongyang propaganda across the rivals' tense border Friday on what was believed to be Kim Jong Un's birthday.

North Korea considers such broadcasts to be an act of psychological warfare and likely will have a furious response. Pyongyang is extremely sensitive to any outside criticism of the authoritarian leadership of Kim, the third member of his family to rule the country. When South Korea briefly resumed propaganda broadcasts in August after an 11-year break, Seoul says the two Koreas exchanged artillery fire, followed by threats of war.

South Korea's Yonhap news agency reported that frontline troops, near 11 sites where loudspeakers started blaring propaganda at noon, were on highest alert. Yonhap said Seoul had deployed missiles, artillery and other weapons systems near the border to swiftly deal with any possible North Korean provocation. South Korea's Defence Ministry did not immediately confirm the reports.

The North Korean response could be harsh because of the high emotions surrounding the likely birthday of Kim, who is believed to be in his early 30s. North Korean military forces often compete to show their loyalty to the leader. The North's state media has yet to mention Kim's birthday or South Korea's loudspeaker campaign.

The broadcasts came as world powers sought to find other ways to punish the North for conducting what it said was its first hydrogen bomb test Wednesday.

U.S. Secretary of State John Kerry urged China, the North's only major ally and its biggest aid provider, to end "business as usual" with North Korea.

Diplomats at a U.N. Security Council emergency session pledged to swiftly pursue new sanctions. For current sanctions and any new penalties to work, better co-operation and stronger implementation from China is seen as key.

South Korean and U.S. military leaders also have discussed the deployment of U.S. "strategic assets," Seoul's Defence Ministry said. Officials refused to elaborate, but the assets likely are B-52 bombers, F-22 stealth fighters and nuclear-powered submarines.

After North Korea's third nuclear test in 2013, the U.S. took the unusual step of sending its most powerful warplanes -- B-2 stealth bombers, F-22 stealth fighters and B-52 bombers -- to drills with South Korea in a show of force. B-2 and B-52 bombers are capable of delivering nuclear weapons.

It may take weeks or longer to confirm or refute the North's claim that it successfully tested a hydrogen bomb, which would mark a major and unanticipated advance for its still-limited nuclear arsenal. Outside experts are skeptical the blast was a hydrogen bomb, but even a test of an atomic bomb would push North Korea closer to building a nuclear warhead small enough to place on a long-range missile.

Late Friday, the Korea Institute of Nuclear Safety said a small amount of radioactive elements was found in air samples collected from the peninsula's eastern seas after the blast but the measured amount was too small to determine whether the North had really detonated a nuclear device.

The institute said the level of xenon-133 isotopes found in the samples was similar to levels normally detected at its two radioactive gas detectors on the eastern and western coasts. KINS official Lee Ki-hyeong also noted that other types of xenon isotopes used to confirm nuclear explosions weren't detected.

Lee said the institute will continue to collect and analyze more samples.

British Foreign Secretary Philip Hammond had asked South Korea to refrain from the propaganda broadcasts. But South Korea sees K-pop and propaganda as quick ways to show its displeasure -- and a guaranteed irritant to the North's sensitive and proud leadership.

The broadcasts include Korean pop songs, world news and weather forecasts as well as criticism of the North's nuclear test, its troubled economy and dire human rights conditions, according to Seoul's Defence Ministry.

Performers on Seoul's propaganda playlist include a female K-pop band that rose to fame when its members fell multiple times on stage, a middle-aged singer who rose from obscurity last year with a song about living for 100 years and songs by a young female singer, IU, whose sweet, girlish voice might be aimed at North Korean soldiers deployed near the border.

North Koreans are prohibited from listening to K-pop, but defectors have said their countrymen enjoy music and other elements of South Korea popular culture that are smuggled into the country on USB sticks and DVDs.

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Photos

A South Korean soldier adjusts equipment used for the propaganda broadcasts at a studio near the border between South Korea and North Korea in Yeoncheon, South Korea, Friday, Jan. 8, 2016. (Lim Tae-hoon / Newsis)